Field of the Invention
The present invention relates to a microphone apparatus for producing a signal representing sound pressure at an arbitrarily determined position.
A plurality of microphones can be arranged in a microphone array to achieve a desired directivity which can be adjusted easily by electrical means and which matches well with signal processing techniques. Accordingly, a substantial amount of research and development has been carried out in this area.
However, where it is sought to achieve a desired directivity while accommodating a wide band of sound frequencies, a relatively large number of microphone elements are employed in the array resulting in an increase in the outer diameter of the array. In the microphone array, sound pressure deviations (particularly, phase differences) exist among the microphone array elements and these differences provide the ability to achieve the formation of a desired directivity. The high frequency limit of the sound frequencies to be controlled determines the intervals between array elements. On the other hand, the spacing between the outermost array elements is determined by the low frequency limits. Therefore, where it is intended to obtain desired directivities over a wide frequency band, a large number of microphone elements must be employed and the outer diameter of the microphone array becomes relatively large.
As an example, a microphone array may be provided having a low frequency limit of 100 Hz and a high frequency limit of 10 kHz. In this example, the spacing between the outermost array elements is set to a value corresponding to one wave length (or one-half wave length) of the low frequency limit, so that in this example a spacing of 3.4 m is selected. On the other hand, the intervals between the adjacent array elements are set to a value corresponding to one wave length (or one-half wave length) of the high frequency limit, so that in this example the array elements are spaced by 3.4 cm. Accordingly, in this exemplary microphone array, a disadvantageously large number of array elements are required and the array must have a relatively large outer diameter.